In modern society, increased electricity demand has brought about increased demand for power cables. In response to this trend, power cables to transport high-capacity power have been actively developed.
In general, a power cable includes a conductor and an insulation layer surrounding the conductor. A high- or super-high-voltage cable may further include an inner semiconductor layer between the conductor and the insulation layer, an outer semiconductor layer surrounding the insulation layer, a sheath layer surrounding the outer semiconductor layer, and the like. The power cable is a consumable product which is used for a given period of time and then disposed. As conventional insulation materials for power cables, crosslinked substances of polyolefin-based polymers such as polyethylene, ethylene/propylene elastomeric copolymers (ethylene/propylene rubbers, EPR) and ethylene/propylene/diene copolymers (EPDM) that have excellent mechanical and electrical properties are generally used.
However, in order to recycle copper (Cu) and aluminum (Al), which are conductors of cables which occupy most of production cost, it is necessary to remove insulation layers that fail to satisfy requirements after production of cables or insulation layers of used cables. However, crosslinked polyethylene (crosslinked PE:XLPE) used as an insulation layer is removed by incineration upon disposal of used cables because reforming (remolding) of XLPE is impossible. At this time, a great amount of CO2 is generated and the burned ash should be buried in landfill, which causes an environmental problem. In addition, so as to produce XLPE cables, a crosslinkage process at high temperature and at high pressure (for example, about 300° C., 7 atm or the like) is required after cable forming (molding). For this purpose, several tens of meters or more of curing tubes should be introduced, which requires considerable production facility site and power costs. Furthermore, addition of expensive chemical substances (crosslinking agents or the like) is necessary for crosslinkage, which disadvantageously results in increased production costs and waste of resources.
Accordingly, a variety of attempts to solve these problems have been made. Some prior art suggests use, as an insulation material, of a composition in which propylene copolymer particles are dispersed in polypropylene as a matrix. In accordance with this technology, an eco-friendly polypropylene resin is used because the polypropylene matrix has a melting point of 150° C. or higher and thus provides excellent heat resistance even without crosslinkage. In addition, in order to improve insufficient flexibility, bendability and the like, which are drawbacks of the polypropylene resin, a composition in which a propylene copolymer having a predetermined mean weight particle size is dispersed in the polypropylene matrix is used as an insulation material. Cable products can be manufactured immediately after molding (forming) cables, without an additional crosslinkage process, which causes considerable reduction of production costs and enables insulation layers that fail to satisfy requirements after production of cables to be recycled by separating and then reforming the insulation layers.
However, the composition has insufficient flexibility and bendability, for example, undergoes a phenomenon, called “stress whitening (blush)”, upon bending of power cables or application of external mechanical stress and tension.
In addition, there is a limitation of low remoldability due to high melting point or the like during a reforming process to recycle used power cables or defective power cable products.
Accordingly, there is an increasing need for polymer compositions for an insulation layer of power cables that satisfy basic properties such as flexibility and bendability comparable to XLPE such that the polymer compositions can replace XLPE, and has excellent remoldability.